The invention relates to the controlled inflation of balloon cuffs which surround the end of tracheal tubes used in respiratory medicine. Tracheal tubes include both tracheostomy and endotracheal tubes. An endotracheal tube is one which can quickly be inserted through the mouth or nose into the trachea, while a tracheostomy tube must be inserted through a surgical opening in the neck. However, for the purposes of this invention, they may be viewed as the same, and will therefore herein be referred to collectively as tracheal tubes, unless otherwise stated.
Tracheal tubes may be inserted for a variety of reasons, including, the need for mechanical ventilation, bypass of an obstruction, removal of secretions, easier ventilation due to less dead space. In most circumstances, it is necessary to seal the outside of the tracheal tube to the inner tracheal lining, i.e., the tracheal mucosa. During mechanical ventilation this is particularly true, since a closed circuit is necessary for a ventilator to force a given volume of air or oxygen under pressure into a patient's lungs. When a patient is not being mechanically ventilated, a seal may or may not be required. In this situation, the need for a seal will generally depend on whether or not there is a risk of aspiration. In the case of an endotracheal tube, this risk is always present, while with a tracheostomy tube, this risk is often present. Therefore, most patients require a seal either to prevent aspiration or to create a closed circuit for mechanical ventilation. Because of the pressurization of the system, a tighter seal is necessary during mechanical ventilation than is necessary to prevent aspiration in the absence of mechanical ventilation.
It is well known that over pressurization of the tracheal cuff can cause significant tracheal damage including, hemorrhage, ulcers, perforation, and strictures. It is generally accepted that the main cause of this damage is occlusion of blood vessels leading to loss of blood flow with resultant necrosis of the tracheal lining. Experience has shown that an intra cuff pressure of less than 25 cm H.sub.2 O is associated with significantly fewer complications. This is consistent with experimental data suggesting that the capillary perfusion pressure in the tracheal mucosa is in the range of 30-40 cm H.sub.2 O. Thus, a cuff pressure of 25 cm H.sub.2 O would still allow some blood flow. However, the numbers noted above are not absolute. Damage is occasionally seen with cuff pressures of 25 cm H.sub.2 O because perfusion pressures may be lower than expected due, for example, to low blood pressure. Therefore, the best approach is to use the lowest cuff pressure consistent with an adequate ventilation seal and the prevention of aspiration.
Tracheal cuffs are frequently positional, that is, as the patient moves, the balloon moves into tighter and looser positions within the trachea. This causes the pressure in the cuff to increase and decrease respectively. The result is too much pressure on the mucosa or a pressure leak, respectively. The problem of tracheal tube movement is not just a theoretical one. The phrases "positional tube" or "positional cuff" are ones frequently heard in practice to report a condition in which, after proper inflation of a cuff, movement of the patient causes excessive, often audible leakage of the ventilator volume around the cuff. In this case, the patient's lungs fail to receive the prescribed volume. Positional cuff occurring during the exhalation phase has the attendant risk of aspiration. The same kind of risk is presented by positional cuff where a tracheal tube is used without a volume ventilator, for example, in a tracheostomy. There is a further way in which deflation can occur: through a leak in the balloon cuff itself or in the cuff inflation line.